Re: How are the craters on Phobos 'eroded'?

All surfaces in the solar system have been impacted by other objects throughout the history of the solar system. When an impact occurs on a solid surface, a crater is produced and appears as a hole. The longer a surface has been exposed to impacts and unaltered by other processes, the more craters it has. Some surfaces, for example the Moon’s surface, have been exposed to impacts for over 4 billion years and have not undergone much modification. These very old surfaces are covered in craters of all sizes. In contrast, Earth’s surface has very few impact craters, because its surface is continually being eroded by water, wind, landslides, volcanism, and tectonic processes, and its oceanic crust (~75% of the Earth’s surface) is continually being subducted (recycled back into the mantle) and reformed (at mid-ocean ridges).

Phobos, the larger of Mars’s moons, is a relatively small object (27 km x 21 km x 19 km in size). The surface of Phobos, as mentioned in the question, is riddled with craters, meaning its surface has been exposed to impacts for billions of years. Geologically, Phobos is not an active body, so these craters have not been modified by volcanism or tectonics. Phobos also has no hydrosphere or atmosphere, so the craters have not been eroded by water or wind.

The Moon has been exposed to impacts for approximately the same amount of time as Phobos. Why, then, do lunar craters appear to be relatively sharp and unweathered, whereas many of Phobos’s craters appear subdued? An additional mystery lies in the number of small craters observed: if you counted the number of small craters in a given area of the Moon and in the same sized area on Phobos, you would count many fewer small craters on Phobos. The reason is not that there were fewer small impacts; it is because they have been erased from the surface.

One way that craters can be eroded on all airless bodies is through other impacts. Sometimes an impact occurs on top of a previously formed crater. Other times, the ejecta from impacts blanket the region surrounding the crater. On a small body like Phobos, a relatively large impact can blanket the entire body with ejecta. Over time, this can build up a substantial regolith (loose, rocky material, similar in concept to soil on Earth) that can slowly cover old craters.

The global blanketing of ejecta will contribute to the weathering of craters, however another process is likely to be more important. This process is known as “seismic shaking”, and is also caused by impacts. The small size of Phobos, along with its low gravity (only 0.5% as strong as Earth’s gravity), mean that impacts can have a global effect. When a large enough object impacts the surface of Phobos, it sends seismic vibrations throughout the whole body. These vibrations destabilize the regolith, and pieces of the regolith move down slopes under the influence of gravity. The process of seismic shaking is actually enhanced by low gravity, because it is easier to move material longer distances. Over time (after many, many impact events), this process will erase small craters by slowly filling them in, and cause larger craters to appear weathered or degraded. In other words, small craters are affected more significantly than large craters.

To understand more what this process looks like, fill a bowl with flour. Create “craters” of different sizes by pressing the surface of the flour with your thumb or round objects (like a ball). If you carefully shake or tap on the side of the bowl, the craters you created would begin to disappear or level out. Smaller craters will disappear faster than larger craters.